PHASE-BEHAVIOR, MORPHOLOGY, AND MECHANICAL-PROPERTIES OF POLYETHYLENE-COPOLYMER BLENDS

Binary blends of unbranched polyethylene (PE) and 5-10% model ethylene -butene random copolymers are used to determine the effects of composi tion heterogeneity on phase separation in the melt, semicrystalline mo rphology, plane strain fracture toughness J(C) and tensile modulus and yield strength. Slowly cooled samples of melt-miscible blends are app reciably tougher (J(C) = 5.2 kJ/m2) than unblended PE (J(C) = 2.7 kJ/m 2). A blend with the same average short chain branch concentration, bu t which is phase separated in the melt state, has J(C) = 3.3 kJ/m2; di spersed domains of amorphous polymer have little effect on toughness. Enhanced toughness is associated with nonuniform morphology formed on slow cooling ''one phase'' melts composed of chains with different amo unts of branching. The relative number of chemically different chains, as opposed to absolute branch concentrations, seems most important. T ensile properties are relatively unaffected by blending at these level s. Results from these model blends are used to consider the properties of compositionally heterogeneous ethylene copolymers. (C) 1994 John W iley & Sons, Inc.